Auto merge of #93368 - eddyb:diagbld-guarantee, r=estebank

rustc_errors: let `DiagnosticBuilder::emit` return a "guarantee of emission".

That is, `DiagnosticBuilder` is now generic over the return type of `.emit()`, so we'll now have:
* `DiagnosticBuilder<ErrorReported>` for error (incl. fatal/bug) diagnostics
  * can only be created via a `const L: Level`-generic constructor, that limits allowed variants via a `where` clause, so not even `rustc_errors` can accidentally bypass this limitation
  * asserts `diagnostic.is_error()` on emission, just in case the construction restriction was bypassed (e.g. by replacing the whole `Diagnostic` inside `DiagnosticBuilder`)
  * `.emit()` returns `ErrorReported`, as a "proof" token that `.emit()` was called
    (though note that this isn't a real guarantee until after completing the work on
     #69426)
* `DiagnosticBuilder<()>` for everything else (warnings, notes, etc.)
  * can also be obtained from other `DiagnosticBuilder`s by calling `.forget_guarantee()`

This PR is a companion to other ongoing work, namely:
* #69426
  and it's ongoing implementation:
  #93222
  the API changes in this PR are needed to get statically-checked "only errors produce `ErrorReported` from `.emit()`", but doesn't itself provide any really strong guarantees without those other `ErrorReported` changes
* #93244
  would make the choices of API changes (esp. naming) in this PR fit better overall

In order to be able to let `.emit()` return anything trustable, several changes had to be made:
* `Diagnostic`'s `level` field is now private to `rustc_errors`, to disallow arbitrary "downgrade"s from "some kind of error" to "warning" (or anything else that doesn't cause compilation to fail)
  * it's still possible to replace the whole `Diagnostic` inside the `DiagnosticBuilder`, sadly, that's harder to fix, but it's unlikely enough that we can paper over it with asserts on `.emit()`
* `.cancel()` now consumes `DiagnosticBuilder`, preventing `.emit()` calls on a cancelled diagnostic
  * it's also now done internally, through `DiagnosticBuilder`-private state, instead of having a `Level::Cancelled` variant that can be read (or worse, written) by the user
  * this removes a hazard of calling `.cancel()` on an error then continuing to attach details to it, and even expect to be able to `.emit()` it
  * warnings were switched to *only* `can_emit_warnings` on emission (instead of pre-cancelling early)
  * `struct_dummy` was removed (as it relied on a pre-`Cancelled` `Diagnostic`)
* since `.emit()` doesn't consume the `DiagnosticBuilder` <sub>(I tried and gave up, it's much more work than this PR)</sub>,
  we have to make `.emit()` idempotent wrt the guarantees it returns
  * thankfully, `err.emit(); err.emit();` can return `ErrorReported` both times, as the second `.emit()` call has no side-effects *only* because the first one did do the appropriate emission
* `&mut Diagnostic` is now used in a lot of function signatures, which used to take `&mut DiagnosticBuilder` (in the interest of not having to make those functions generic)
  * the APIs were already mostly identical, allowing for low-effort porting to this new setup
  * only some of the suggestion methods needed some rework, to have the extra `DiagnosticBuilder` functionality on the `Diagnostic` methods themselves (that change is also present in #93259)
  * `.emit()`/`.cancel()` aren't available, but IMO calling them from an "error decorator/annotator" function isn't a good practice, and can lead to strange behavior (from the caller's perspective)
  * `.downgrade_to_delayed_bug()` was added, letting you convert any `.is_error()` diagnostic into a `delay_span_bug` one (which works because in both cases the guarantees available are the same)

This PR should ideally be reviewed commit-by-commit, since there is a lot of fallout in each.

r? `@estebank` cc `@Manishearth` `@nikomatsakis` `@mark-i-m`

compiler/rustc_borrowck/src/borrowck_errors.rs

@@ -1,9 +1,13 @@
-use rustc_errors::{struct_span_err, DiagnosticBuilder, DiagnosticId};
+use rustc_errors::{struct_span_err, DiagnosticBuilder, DiagnosticId, ErrorReported};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_span::{MultiSpan, Span};
 
 impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
-    crate fn cannot_move_when_borrowed(&self, span: Span, desc: &str) -> DiagnosticBuilder<'cx> {
+    crate fn cannot_move_when_borrowed(
+        &self,
+        span: Span,
+        desc: &str,
+    ) -> DiagnosticBuilder<'cx, ErrorReported> {
         struct_span_err!(self, span, E0505, "cannot move out of {} because it is borrowed", desc,)
     }
 
@@ -13,7 +17,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         desc: &str,
         borrow_span: Span,
         borrow_desc: &str,
-    ) -> DiagnosticBuilder<'cx> {
+    ) -> DiagnosticBuilder<'cx, ErrorReported> {
         let mut err = struct_span_err!(
             self,
             span,
@@ -32,7 +36,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         span: Span,
         verb: &str,
         desc: &str,
-    ) -> DiagnosticBuilder<'cx> {
+    ) -> DiagnosticBuilder<'cx, ErrorReported> {
         struct_span_err!(
             self,
             span,
@@ -51,7 +55,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         old_loan_span: Span,
         old_opt_via: &str,
         old_load_end_span: Option<Span>,
-    ) -> DiagnosticBuilder<'cx> {
+    ) -> DiagnosticBuilder<'cx, ErrorReported> {
         let via =
             |msg: &str| if msg.is_empty() { "".to_string() } else { format!(" (via {})", msg) };
         let mut err = struct_span_err!(
@@ -99,7 +103,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         desc: &str,
         old_loan_span: Span,
         old_load_end_span: Option<Span>,
-    ) -> DiagnosticBuilder<'cx> {
+    ) -> DiagnosticBuilder<'cx, ErrorReported> {
         let mut err = struct_span_err!(
             self,
             new_loan_span,
@@ -132,7 +136,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         noun_old: &str,
         old_opt_via: &str,
         previous_end_span: Option<Span>,
-    ) -> DiagnosticBuilder<'cx> {
+    ) -> DiagnosticBuilder<'cx, ErrorReported> {
         let mut err = struct_span_err!(
             self,
             new_loan_span,
@@ -164,7 +168,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         old_opt_via: &str,
         previous_end_span: Option<Span>,
         second_borrow_desc: &str,
-    ) -> DiagnosticBuilder<'cx> {
+    ) -> DiagnosticBuilder<'cx, ErrorReported> {
         let mut err = struct_span_err!(
             self,
             new_loan_span,
@@ -200,7 +204,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         kind_old: &str,
         msg_old: &str,
         old_load_end_span: Option<Span>,
-    ) -> DiagnosticBuilder<'cx> {
+    ) -> DiagnosticBuilder<'cx, ErrorReported> {
         let via =
             |msg: &str| if msg.is_empty() { "".to_string() } else { format!(" (via {})", msg) };
         let mut err = struct_span_err!(
@@ -243,7 +247,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         span: Span,
         borrow_span: Span,
         desc: &str,
-    ) -> DiagnosticBuilder<'cx> {
+    ) -> DiagnosticBuilder<'cx, ErrorReported> {
         let mut err = struct_span_err!(
             self,
             span,
@@ -262,20 +266,20 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         span: Span,
         desc: &str,
         is_arg: bool,
-    ) -> DiagnosticBuilder<'cx> {
+    ) -> DiagnosticBuilder<'cx, ErrorReported> {
         let msg = if is_arg { "to immutable argument" } else { "twice to immutable variable" };
         struct_span_err!(self, span, E0384, "cannot assign {} {}", msg, desc)
     }
 
-    crate fn cannot_assign(&self, span: Span, desc: &str) -> DiagnosticBuilder<'cx> {
+    crate fn cannot_assign(&self, span: Span, desc: &str) -> DiagnosticBuilder<'cx, ErrorReported> {
         struct_span_err!(self, span, E0594, "cannot assign to {}", desc)
     }
 
     crate fn cannot_move_out_of(
         &self,
         move_from_span: Span,
         move_from_desc: &str,
-    ) -> DiagnosticBuilder<'cx> {
+    ) -> DiagnosticBuilder<'cx, ErrorReported> {
         struct_span_err!(self, move_from_span, E0507, "cannot move out of {}", move_from_desc,)
     }
 
@@ -287,7 +291,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         move_from_span: Span,
         ty: Ty<'_>,
         is_index: Option<bool>,
-    ) -> DiagnosticBuilder<'cx> {
+    ) -> DiagnosticBuilder<'cx, ErrorReported> {
         let type_name = match (&ty.kind(), is_index) {
             (&ty::Array(_, _), Some(true)) | (&ty::Array(_, _), None) => "array",
             (&ty::Slice(_), _) => "slice",
@@ -309,7 +313,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         &self,
         move_from_span: Span,
         container_ty: Ty<'_>,
-    ) -> DiagnosticBuilder<'cx> {
+    ) -> DiagnosticBuilder<'cx, ErrorReported> {
         let mut err = struct_span_err!(
             self,
             move_from_span,
@@ -327,7 +331,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         verb: &str,
         optional_adverb_for_moved: &str,
         moved_path: Option<String>,
-    ) -> DiagnosticBuilder<'tcx> {
+    ) -> DiagnosticBuilder<'tcx, ErrorReported> {
         let moved_path = moved_path.map(|mp| format!(": `{}`", mp)).unwrap_or_default();
 
         struct_span_err!(
@@ -346,7 +350,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         span: Span,
         path: &str,
         reason: &str,
-    ) -> DiagnosticBuilder<'cx> {
+    ) -> DiagnosticBuilder<'cx, ErrorReported> {
         struct_span_err!(self, span, E0596, "cannot borrow {} as mutable{}", path, reason,)
     }
 
@@ -357,7 +361,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         immutable_place: &str,
         immutable_section: &str,
         action: &str,
-    ) -> DiagnosticBuilder<'cx> {
+    ) -> DiagnosticBuilder<'cx, ErrorReported> {
         let mut err = struct_span_err!(
             self,
             mutate_span,
@@ -376,7 +380,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         &self,
         span: Span,
         yield_span: Span,
-    ) -> DiagnosticBuilder<'cx> {
+    ) -> DiagnosticBuilder<'cx, ErrorReported> {
         let mut err = struct_span_err!(
             self,
             span,
@@ -387,7 +391,10 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         err
     }
 
-    crate fn cannot_borrow_across_destructor(&self, borrow_span: Span) -> DiagnosticBuilder<'cx> {
+    crate fn cannot_borrow_across_destructor(
+        &self,
+        borrow_span: Span,
+    ) -> DiagnosticBuilder<'cx, ErrorReported> {
         struct_span_err!(
             self,
             borrow_span,
@@ -400,7 +407,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         &self,
         span: Span,
         path: &str,
-    ) -> DiagnosticBuilder<'cx> {
+    ) -> DiagnosticBuilder<'cx, ErrorReported> {
         struct_span_err!(self, span, E0597, "{} does not live long enough", path,)
     }
 
@@ -410,7 +417,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         return_kind: &str,
         reference_desc: &str,
         path_desc: &str,
-    ) -> DiagnosticBuilder<'cx> {
+    ) -> DiagnosticBuilder<'cx, ErrorReported> {
         let mut err = struct_span_err!(
             self,
             span,
@@ -435,7 +442,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         closure_kind: &str,
         borrowed_path: &str,
         capture_span: Span,
-    ) -> DiagnosticBuilder<'cx> {
+    ) -> DiagnosticBuilder<'cx, ErrorReported> {
         let mut err = struct_span_err!(
             self,
             closure_span,
@@ -454,11 +461,14 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
     crate fn thread_local_value_does_not_live_long_enough(
         &self,
         span: Span,
-    ) -> DiagnosticBuilder<'cx> {
+    ) -> DiagnosticBuilder<'cx, ErrorReported> {
         struct_span_err!(self, span, E0712, "thread-local variable borrowed past end of function",)
     }
 
-    crate fn temporary_value_borrowed_for_too_long(&self, span: Span) -> DiagnosticBuilder<'cx> {
+    crate fn temporary_value_borrowed_for_too_long(
+        &self,
+        span: Span,
+    ) -> DiagnosticBuilder<'cx, ErrorReported> {
         struct_span_err!(self, span, E0716, "temporary value dropped while borrowed",)
     }
 
@@ -467,7 +477,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         sp: S,
         msg: &str,
         code: DiagnosticId,
-    ) -> DiagnosticBuilder<'tcx> {
+    ) -> DiagnosticBuilder<'tcx, ErrorReported> {
         self.infcx.tcx.sess.struct_span_err_with_code(sp, msg, code)
     }
 }
@@ -476,7 +486,7 @@ crate fn borrowed_data_escapes_closure<'tcx>(
     tcx: TyCtxt<'tcx>,
     escape_span: Span,
     escapes_from: &str,
-) -> DiagnosticBuilder<'tcx> {
+) -> DiagnosticBuilder<'tcx, ErrorReported> {
     struct_span_err!(
         tcx.sess,
         escape_span,

compiler/rustc_borrowck/src/diagnostics/bound_region_errors.rs

@@ -1,4 +1,4 @@
-use rustc_errors::DiagnosticBuilder;
+use rustc_errors::{DiagnosticBuilder, ErrorReported};
 use rustc_infer::infer::canonical::Canonical;
 use rustc_infer::infer::error_reporting::nice_region_error::NiceRegionError;
 use rustc_infer::infer::region_constraints::Constraint;
@@ -120,7 +120,11 @@ impl<'tcx, F, G> ToUniverseInfo<'tcx> for Canonical<'tcx, type_op::custom::Custo
 trait TypeOpInfo<'tcx> {
     /// Returns an error to be reported if rerunning the type op fails to
     /// recover the error's cause.
-    fn fallback_error(&self, tcx: TyCtxt<'tcx>, span: Span) -> DiagnosticBuilder<'tcx>;
+    fn fallback_error(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        span: Span,
+    ) -> DiagnosticBuilder<'tcx, ErrorReported>;
 
     fn base_universe(&self) -> ty::UniverseIndex;
 
@@ -130,7 +134,7 @@ trait TypeOpInfo<'tcx> {
         cause: ObligationCause<'tcx>,
         placeholder_region: ty::Region<'tcx>,
         error_region: Option<ty::Region<'tcx>>,
-    ) -> Option<DiagnosticBuilder<'tcx>>;
+    ) -> Option<DiagnosticBuilder<'tcx, ErrorReported>>;
 
     fn report_error(
         &self,
@@ -188,7 +192,11 @@ struct PredicateQuery<'tcx> {
 }
 
 impl<'tcx> TypeOpInfo<'tcx> for PredicateQuery<'tcx> {
-    fn fallback_error(&self, tcx: TyCtxt<'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
+    fn fallback_error(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        span: Span,
+    ) -> DiagnosticBuilder<'tcx, ErrorReported> {
         let mut err = tcx.sess.struct_span_err(span, "higher-ranked lifetime error");
         err.note(&format!("could not prove {}", self.canonical_query.value.value.predicate));
         err
@@ -204,7 +212,7 @@ impl<'tcx> TypeOpInfo<'tcx> for PredicateQuery<'tcx> {
         cause: ObligationCause<'tcx>,
         placeholder_region: ty::Region<'tcx>,
         error_region: Option<ty::Region<'tcx>>,
-    ) -> Option<DiagnosticBuilder<'tcx>> {
+    ) -> Option<DiagnosticBuilder<'tcx, ErrorReported>> {
         tcx.infer_ctxt().enter_with_canonical(
             cause.span,
             &self.canonical_query,
@@ -231,7 +239,11 @@ impl<'tcx, T> TypeOpInfo<'tcx> for NormalizeQuery<'tcx, T>
 where
     T: Copy + fmt::Display + TypeFoldable<'tcx> + 'tcx,
 {
-    fn fallback_error(&self, tcx: TyCtxt<'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
+    fn fallback_error(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        span: Span,
+    ) -> DiagnosticBuilder<'tcx, ErrorReported> {
         let mut err = tcx.sess.struct_span_err(span, "higher-ranked lifetime error");
         err.note(&format!("could not normalize `{}`", self.canonical_query.value.value.value));
         err
@@ -247,7 +259,7 @@ where
         cause: ObligationCause<'tcx>,
         placeholder_region: ty::Region<'tcx>,
         error_region: Option<ty::Region<'tcx>>,
-    ) -> Option<DiagnosticBuilder<'tcx>> {
+    ) -> Option<DiagnosticBuilder<'tcx, ErrorReported>> {
         tcx.infer_ctxt().enter_with_canonical(
             cause.span,
             &self.canonical_query,
@@ -288,7 +300,11 @@ struct AscribeUserTypeQuery<'tcx> {
 }
 
 impl<'tcx> TypeOpInfo<'tcx> for AscribeUserTypeQuery<'tcx> {
-    fn fallback_error(&self, tcx: TyCtxt<'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
+    fn fallback_error(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        span: Span,
+    ) -> DiagnosticBuilder<'tcx, ErrorReported> {
         // FIXME: This error message isn't great, but it doesn't show up in the existing UI tests,
         // and is only the fallback when the nice error fails. Consider improving this some more.
         tcx.sess.struct_span_err(span, "higher-ranked lifetime error")
@@ -304,7 +320,7 @@ impl<'tcx> TypeOpInfo<'tcx> for AscribeUserTypeQuery<'tcx> {
         cause: ObligationCause<'tcx>,
         placeholder_region: ty::Region<'tcx>,
         error_region: Option<ty::Region<'tcx>>,
-    ) -> Option<DiagnosticBuilder<'tcx>> {
+    ) -> Option<DiagnosticBuilder<'tcx, ErrorReported>> {
         tcx.infer_ctxt().enter_with_canonical(
             cause.span,
             &self.canonical_query,
@@ -329,7 +345,7 @@ fn try_extract_error_from_fulfill_cx<'tcx>(
     infcx: &InferCtxt<'_, 'tcx>,
     placeholder_region: ty::Region<'tcx>,
     error_region: Option<ty::Region<'tcx>>,
-) -> Option<DiagnosticBuilder<'tcx>> {
+) -> Option<DiagnosticBuilder<'tcx, ErrorReported>> {
     let tcx = infcx.tcx;
 
     // We generally shouldn't have errors here because the query was